The quantity of oil and gas production from a hydrocarbon bearing strata into a borehole is influenced by many physical factors. Darcey's flow equation, which defines flow in a well, takes into account the reservoir constants of temperature, viscosity, permeability, reservoir pressure, pressure in the borehole, thickness of the producing strata, and the area exposed to flow.
It has long been known that increasing the exposed flow area in a producing well increases production. For example, it is known that drilling a larger diameter hole exposes more of the producing strata and thus increases production.
Enlarging the flow areas, in open hole intervals, has been accomplished by using both explosives and chemicals. However, use of these agents is somewhat limited where the producing strata are cemented behind steel casing. In cased applications, the well is “perforated” to create small holes that extend through the steel casing, the annulus cement and the adjacent formation.
Prior to the invention of the shaped charge, wells were perforated with multiple, short-barreled guns. The bullets penetrated the casing, the annulus cement, and the producing strata. The shaped charge, with its greater penetration and reliability, though, has largely replaced the so-called “bullet guns.”
A shaped charge makes a hole through the casing and into the strata by forming a high speed stream of particles that are concentrated in a small diameter jet. As the high energy particles hit solid material, the solid material is pulverized. Thus, shaped charges can be used to place numerous small perforations where desired in a well. However, the fine material from the pulverized rock and the shaped charge particles can have a detrimental effect on fluid flow in the area around the perforation. Debris from the spent charge as well as fragments and particles from the pulverized formation tend to plug the perforations and obstruct passages in the fractured formation.
The formation pressure acts on the small oil droplets in the formation to force the hydrocarbons from the connected pore spaces into the well bore. The magnitude of the area in the formation exposed by the perforations directly affects the amount of flow and/or work required for that production. Accordingly, increasing the exposed flow area by perforation does two favorable things: it increases the flow rate directly, and, it reduces the amount of work required to maintain a given production rate. Increasing the flow area in a well increases the ultimate recovery from the well/reservoir by conserving formation pressure or reservoir energy.
The present invention provides a method and apparatus capable of increasing the exposed surface area in a formation when using shaped charges to perforate a well. This apparatus and method augment the use of shaped charges by including the introduction of oxygen rich material into the formation with the explosive. The delivery of an oxygen source to the hydrocarbon-containing formation, in the presence of the explosive reaction, provides sustained explosive burning of the hydrocarbons in the vicinity of the perforation. The burning in the formation continues until the oxygen-rich material is depleted, when the burning self-extinguishes. Thus, the extent of the burning can be controlled by selecting the amount of oxygen-rich material to be introduced into the formation.
This significant secondary reaction in the strata has two beneficial effects. In the first place, the reaction will cause a cleaning effect on the fine particles that might otherwise plug the perforation. The cleaning effect occurs when the explosive burning causes high pressure gases to be generated, and these pressurized gases are discharged rapidly back into the borehole or casing. Secondly, the extended burning or explosion in the treated statum causes further fracturing of the formation. This results in further expansion of the exposed flow areas in the formation beyond the initial shaped charge perforation. In addition, in the event the strata being perforated are water bearing, the explosive reaction will not occur; rather, only oil or gas bearing formations will be stimulated.